The use of rheology modification agents, frequently thickening agents, for aqueous and hydrophylic fluids has been common practice in a large number of industries. These fluids include, for example, oil field drilling fluids, metal-working fluids, mining fluids, fire control fluids, hydraulic fluids, water-based paints and coating fluids, stripping fluids, and the like. For each of these, and other, applications, the rheology modification agents serve very specific purposes tailored to the function for which the fluid is being employed. Among these purposes are pressure resistance; suspension of solids; adjustment of reaction time(s); protection against temperature extremes or variations; durability and resistance to degradation under conditions of use; protection from undesirable external forces such as bacterial attack, oxidation, or chemical reaction such as corrosion; and the like. Because a variety of specific chemical and/or physical properties is frequently desired for a given application, the rheology modification agent has frequently been used in conjunction with other types of agents or additives, in order to produce a final fluid suitable to a given application. However, since it is generally desirable to reduce the number of such agents or additives as much as possible, in order to facilitate the ease of production and use and, therefore, to also minimize cost, it is desirable to employ a rheology modification agent which offers the greatest number of benefits to the fluid for its intended use.
A variety of general rheology modification agents are known and have been qualified for use in various specific applications. For example, polymeric materials such as xanthan gum, guar gum and polyacrylamides have historically been used as rheology modification agents in water-based drilling fluids, but have been found to be unstable in the presence of various salts encountered in some formations and in subsea drillsites. These materials also tend to exhibit undesirable susceptibility to oxidation and bacterial attack; to degradation when exposed to the shear forces exerted in the drilling process; and/or to thermal degradation above about 250 to 300° C. They also have limited ability to maintain solids suspension upon elimination of shear forces such as those produced during pumping.
Showing better thermal stability are some of the non-polymeric materials, typically clays such as bentonite and attapulgite. For example, bentonite is relatively stable to temperature and offers the additional benefits of resistance oxidation and durability when exposed to high shear conditions. These mineral clays are often used with other types of agents or densifiers, such as iron oxide or barium sulfate, which enhance the ability of the fluid to resist pressures such as are encountered in subterranean excavations.
Unfortunately, the mineral clays, though historically popular, are not without their drawbacks for many applications. Fluids containing bentonite, though probably the most popular of the clay materials for drilling muds, are severely compromised in the presence of polyvalent cations, such as calcium and magnesium, frequently present in drilling formations, and may become so thick at higher temperatures under some circumstances that thinners must frequently be added. Other clay systems also suffer from undesirable reactivity and temperature degradation, and may not be adequately consistent in composition from batch to batch.
Combinations of clays and polymeric materials have also been employed, with the goal of extending the clay and thereby using less of it. Thus, the complexity of the composition is increased and therefore its cost and/or difficulty of preparation, particularly under field conditions. Typical extenders useful with bentonite systems include polyacrylamide. Unfortunately, the weaknesses of the extending polymer, such as thermal instability and the like, may then dominate the characteristics of the fluid as a whole.
In response to the above-cited problems, those skilled in the art have developed a number of newer agents based on hydrous aluminum compounds. In many cases the hydrous aluminum compounds must be formed in situ. This method of preparation results in formation of relatively large amounts of reaction salts which may then cause corrosion of metals such as drill bits, or may undesirably interfere with other performance additives. Furthermore, control of the reaction for in situ preparation may be extremely difficult, depending upon the final application, for example, in the mud pit of a drilling rig.
Also used with some success for rheology modification are crystalline layered mixed metal hydroxides, wherein Li, Mg, Cu, Zn, Mn, Fe, Co, and Ni are part of the layered structure; and also other metal aluminates. Of particular note are the “gelling” materials disclosed in U.S. Pat. Nos. 4,664,843 and 4,790,954, which disclose agents that exhibit not only thickening in general, but also a variable rheology defined as “pseudoplasticity”. Such rheology is characterized by an ability to flow, to a determinable extent, upon exertion of a given shear force, such as that exerted by an actively revolving drill bit or during pumping from or into the mud pit of a drilling rig. The treated fluid then returns to a significantly higher, and again determinable, viscosity when the shear force is removed. These newer rheology modification agents thus are particularly well-suited to solids suspension combined with ease of use. However, in many cases they still suffer from some of the problems associated with the polymeric, clay and combination agents, such as limited inhibition of reactivity with some cations, undesirable toxicity, temperature limitations, insufficient lubricity, and the like. In particular, many of these agents are extremely expensive and thus impractical for drilling-scale applications in particular.
It would therefore be highly useful in the field to identify a family of agents which impart rheology modification to aqueous systems, such that their viscosity levels, with or without application of shear forces, can be optimized at each point in time according to the desired application; which exhibit desirable temperature resistance, lubricity, inhibition of reactivity, and resistance to geological formation pressure; and which are not cost-prohibitive for large scale application.